1. Field of the Invention
The present invention relates generally to power distribution equipment and, more particularly, to an electrical disconnect apparatus for use in an AC secondary network system.
2. Description of the Related Art
Two primary objectives of the electric utility industry in the delivery of electrical power are safety and reliability. Since the late 1920s, AC secondary network systems have been used in certain locations such as downtown business districts and commercial areas in order to provide a high degree of service continuity. In such an AC secondary network system, a plurality of secondary mains surround the area being served, such as a city block, and are connected with a set of bus bars to form a secondary network grid at low voltage to which the customer loads are connected.
Electrical power is supplied to the secondary network by a plurality of medium voltage distribution lines. Each medium voltage distribution line delivers power to the network through network transformers. The transformers reduce the medium voltage from the distribution lines to a lower voltage suitable for distribution to the customers.
In such secondary network systems, a failure of any one distribution line will not result in an interruption of service to the customers since electrical power will be supplied to the customers over the remaining distribution lines. When a failure or fault occurs in a medium voltage distribution line or in one of its associated network transformers, the station end of the distribution line, that is, the end of the distribution line closest to the generating station, is disconnected from the system by opening a feeder circuit breaker. In addition, it is necessary that all of the network transformers on the failed distribution line be disconnected from the secondary network by some type of protective device to prevent power from the secondary network from being fed back through the network transformers to the fault. The protective device that has been used for such purpose is the network protector.
The network protector consists of a specially designed circuit breaker with a closing and opening mechanism that is controlled by a relay. When the network protector is closed, the relay operates to trip the network protector upon a reversal of power flow. The relay acts to close the network protector when, an only when, the proper voltage conditions exist across the network protector.
Network protectors typically have been located outdoors either above ground or below ground and thus have been protected by a sealed enclosure. When a network protector is approached for maintenance, testing, or repair, the network protector must be electrically and physically disconnected from the power distribution equipment on both the network transformer side and the secondary network side. Historically, this consideration dictated the use of a rollout-type or draw-out-type circuit breaker which could be disconnected and rolled out of its enclosure for maintenance, testing, and repair.
However, such a rollout-type or draw-out-type circuit breaker will typically still be energized in some fashion by the attached network, which results in a certain level of potential danger to a technician. It thus would be desirable to alleviate the dangers to technicians and others.